Lining underground slurry trenches with impervious films such as rubber liners, plastic sheets, foils and the like has been attempted prior to this invention but they have not been particularly successful. (See page 130 of R. G. H. Boyers' text entitled "Strutural and Cut-Off Diaphragm Walls" John Waley & Sons). In Ranney Pat. No. 2,048,710, an apparatus is disclosed for constructing an underground wall in which two rolls of lining material are carried inside wings of an excavating shield for unrollling the lining material to line the walls of the trench preparatory to filling of the trench with the desired wall material. There is no teaching in Ranney of utilizing a lining material which is folded (or otherwise joined) at the bottom and of sufficient height to comfortably drape over the sides of the trench. A feature of the process of the present invention is the use of the fill material to weight the lining material and displace the bentonite (using the lining material to transmit the displacing force to the bentonite). In Zaklewicz Pat. No. 3,603,099 and Caron et al Pat. No. 3,759,044, a plastic sheet is immersed in a bentonic mud filled trench. In the Zaklewicz patent the filling material is added to each side of the plastic membrane to displace the excavating slurry and in the Caron et al patent, the plastic sheeting is immersed in a cementious bentonite filled trench and remains there until the wall material has set. Neither of these prior art references disclose the concept of applicant's double or folded plastic membrane nor is there disclosed a membrane of sufficient vertical height to drape over the sides of the trench. In the present invention, the double layer end fold results in a basic wall structure not disclosed in these prior art references.
The use of the lining for protecting underground pipes and conduits has been known in the art as is disclosed in Grodsky Patent No. 2,007,969 and in Keene Pat. No. 3,675,432. However, these are non-analogous to the present invention and neither of these is disclosed in the context of a slurry trench excavating process and obviously, do not in any sense teach the depth of the wall and process as disclosed herein and do not teach or suggest a membrane of 30 to 40 feet in depth containing industrial pollutants or impounding water or other flowable materials. In Grether et al Pat. Nos. 3,298,183 3,218,810 and 3,182,459, a fluid barrier is disclosed for water impoundment and/or channelization but there is no underground structure contemplated, and the double folded material is simply folded at the top above the ground and burried to anchor same against the force of the upstream water.
The basic object of the present invention is to provide an improved impervious underground wall structure and an improved method and apparatus for constructing same, particularly for pollution control.
A basic feature of the invention is that the liner is a folded plastic liner (forming a sheath or envelope for the backfill material) of sufficient height so that both sides thereof can extend and drape over the sides of the slurry filled trench so that the backfill material, which, preferrably, is hydraulically placed and of greater density than the slurry in the trench, carries the fold line to the bottom of the trench and displaces the bentonite slurry towards the end of the excavation where the excavation is taking place thereby reducing the amount of slurry needed. The double layer plastic is unrolled along the trench, the end extending up above the surface is opened up and the backfill material is hydraulically placed between the sheets. Typically the backfill material may be a mixture of sand and water which will have two effects. It will sink the plastic to the very bottom of the trench and make it adhere to the sides of the trench and will displace forward the bentonite, reducing the need for a lot of bentonite for excavation. The wall structure resulting from this technique is novel in that the impervious barrier that is constructed in the earth will consist of an area of soil adjacent the excavation which has been permeated by bentonite (e.g. the side walls of the trench), a bentonite cake, the plastic membrane, the backfill material inside the plastic membrane (typically sand or coarse grannular material) and the same system repeated again on the opposite trench wall side.
The advantages of the system are that it assures an absolute water tightness by two identical pollutant barrier systems in series,--the soil permeated bentonite, the bentonite cake and the plastic membrane. Moreover, the backfill material is used to sink the plastic sheath or envelope to the bottom of the trench, hence, once the initial insertion in the trench of the roll and a preliminary backfilling is accomplished, the plastic material is unrolled or unfolded from an accordion fold and floats in the trench or slot until it is opened up at the top and the backfill material is inserted and the bentonite slurry is displaced to the opposite end of the trench or slot.